1. Field of the Invention
This invention relates to a photographing unit for a microscope.
2. Related Background Art
FIG. 12 is a view for explaining optical paths of a microscopic unit which is provided with a conventional photographing unit for a microscope, while FIG. 13 is a view taken along the line XIII--XIII in FIG. 12.
A microscopic unit 100 is provided with an objective lens 101, a projection lens 102, a photographing unit 110 for a microscope, and a camera 130.
The camera 130 is mounted on the photographing unit 110 for a microscope, in which a film 131 is loaded.
The photographing unit 110 is provided with prism 111 which divides light from a sample 103 which is guided by the objective lens 101, into an optical path A of a photographing system, an optical path B of an observation system and an optical path C of a photometric system.
A shutter 112 is disposed between the camera 130 on the optical path A of the photographing system and the prism 111. This shutter 112 controls an exposure time onto the film 131 of an image of the sample 103 which is formed on a primary image plane 101a of the objective lens 101 and enlarged by the projection lens 102.
On the optical path B of the observation system there are provided a reduction lens 114 which incorporates an angle of view of the film 131 into the field of view of a viewfinder eyepiece 113, and a reticle 115 which has double cross lines serving as an index for a photographing range frame or a focusing operation. The viewfinder eyepiece 113 is used to observe the reticle 115 and an image 115a formed on the reticle 115.
On the optical path C of the photometric system there is provided a photometric detector 123 which measures the brightness of the image from the sample 103.
An operation of the above-mentioned photographing unit for a microscope will be described below.
The prism 111 is located at a-1 position in FIG. 13 at a time other than the time of exposure. A light reflected on an optical path dividing plane RF1 is guided to the viewfinder optical path B, and the light passing through this optical path dividing plane RF1 is reflected on an optical path dividing plane RF2 and is guided to the optical path C of the photometric system.
A photographer adjusts the position of the eyepiece 113 in the direction of the optical axis with respect to the reticle 115 in accordance with a diopter, adjusts the focus on the double cross lines on the reticle 115, and at the same time, moves a stage 104 on which the sample 103 is mounted in the direction of the optical axis so that the double cross lines of the reticle 115 and the image of the sample 103 can be seen clearly.
In the photographing unit 110 for a microscope, an amount of the light which is guided by RF2 to the optical path C of the photometric system is measured by the photometric detector 123, and an exposure time is calculated from this measured value, and a film sensitivity and an exposure correction value which are previously determined.
During exposure, the prism 111 is moved from the position on the solid line in FIG. 13 to the a-2 position shown as the double-dotted line, as indicated by the arrow a, so as to be removed from the optical path B of the observation system and the optical path C of the photometric system. Thus, all of the light from the sample 103 is guided to the optical path A of the photographing system. Then, the shutter 112 is opened only for the exposure time which is determined by the arithmetic calculation, so that the image is projected to the film 131.
Since the photographing for a microscope is usually conducted by enlarging a small portion so that, it is susceptible to vibration. As a result, an obtained photograph may be blurred in some cases due to an instantaneous vibration which is caused when the shutter 112 inside the photographing unit 110 is actuated. In order to avoid the influence of this vibration, the illumination light onto the sample 103 may be darkened, so that, out of the overall exposure time a time period after the mitigation of the vibration is prolonged as compared with a time period in which the operation is under the influence of the initial vibration.
However, when the exposure time is too long, the obtained photograph tends to be underexposed owing to the reciprocity law failure characteristic of the film 131 or to lack a color balance. In such cases, it is necessary to make exposure correction, dispose a filter for color correction in an illumination optical path of the microscope, or take some other counter steps. It is also necessary to retake photographs several times or take a lot of photographs under different conditions, in order to determine appropriate exposure correction or color correction.
Accordingly, in photographing for a microscope, when the exposure time is too short, the photographing is susceptible to vibration. On the other hand, if the exposure time is too long, an obtained photograph is underexposed or lacks a color balance. As a result, in order to obtain an excellent photograph, it is necessary to set such exposure time which can avoid the influence of the vibration of the shutter 112 or the reciprocity law failure characteristic of the film 131.
However, in the conventional photographing unit 110 for a microscope, even when an appropriate value as can avoid the influence of the vibration of the shutter 112 or the reciprocity law failure characteristic of the film 131 is set for the exposure time, if a ratio of division of the prism 111 toward the optical path B of the observation system is set to be small, an image of a viewfinder VF is dark when the sample 103 is dark. On the otherhand, if the ratio of division of the prism 111 toward the optical path B of the observation system is set to be large, the image of the viewfinder VF is too bright so as to be dazzling when the sample 103 is bright.
When the film 131 to be used is of high sensitivity, the exposure time is appropriate with a dark photographed image so that an image of the viewfinder VF at that time becomes dark. When the film 131 to be used is of low sensitivity, since the exposure time does not become appropriate unless a photographed image is bright, the image of the viewfinder VF becomes too bright so as to be dazzling.
Microscope manufacturers have set a ratio of division of the prism 111 in such a manner that an image of the viewfinder VF has reasonable brightness when the exposure time is set to be proper based on a film sensitivity which is most frequently used in general.
However, since a ratio of sensitivity of a low sensitivity film to a high sensitivity film comes to one to several hundreds, if the microscope makers use a film 131 having a sensitivity widely different from the film sensitivity used for determining the ratio of division of the prism 111, the image of the viewfinder VF may be too bright or too dark even when a proper value is set for the exposure time. Thus, the image becomes difficult to observe for the photographer.
For this reason, it is required for the photographer to first adjust an illumination light of the microscope, confirm a photographing range, and conduct focusing for the purpose of obtaining a proper light amount for the viewfinder VF, then to adjust an amount of the illumination light of the microscope for the second time for the purpose of obtaining a proper value for the exposure time. These procedures are troublesome for the photographer.
When light from the sample 103 is weak and it is bright outside the microscope, an amount of a light intruding from the optical path B of the observation system is relatively larger than an amount of the light from the sample 103, so that the photometric detector 123 can no longer conduct a correct photometric operation of the sample 103. As a result, when a photograph is taken with an exposure time which is calculated based on this photometric value, the obtained photograph is often underexposed.
Then, when a correct exposure is required, a cap (not shown) serving as a light-shielding member is put on the tip end of the viewfinder VF to prevent external intrusion of light into the microscopic unit 100, and photometry and exposure are conducted in this state.
However, since an operation for setting the photographing range or focusing is usually conducted for each frame of the film, it is necessary, but troublesome, to attach and remove the cap for each frame. In order to avoid this troublesome operation, a dedicated driving mechanism may be arranged for inserting or retracting the light-shielding member into or from an optical path. However, this brings about a problem that the microscopic unit 100 becomes expensive.
Moreover, since the light from the sample 103 is divided by the prism 111 into the viewfinder VF and the photometric detector 123, an amount of the light to be guided to the photometric detector 123 is less than that of the light to be guided to the film 131 during exposure. Accordingly, when the sample 103 is dark, an amount of the light to be guided to the photometric detector 123 sometimes does not reach the minimum light amount required for satisfying a photometric performance of the photometric detector 123, so that photometry with high reliability can not be conducted.
Further, in the conventional photographing unit for a microscope, when the sample image to be photographed is dark, an index of a reticle which is provided in a viewfinder portion is difficult to be recognized. Therefore, there is known a device for making the index portion more visible by illuminating the reticle. As such device for illuminating the reticle in the conventional photographing unit for a microscope, there is a device which is provided with a plurality of light sources for emitting lights of different colors so as to select any color for illumination, or a device in which an ON/OFF operation for illumination can be arbitrarily conducted by an observer of the microscope.
However, the reticle has to be illuminated by the observer properly, and the reticle illumination tends to be forgotten as lighted or extinguished when samples or observing conditions are changed over. In addition, there are cases in which the illumination light for illuminating the reticle becomes a stray light when a dark object is photographed with automatic exposure, to effect a photometric detector for measuring a light from the sample, thereby rendering an inappropriate exposure, and in which when a bright object is photographed with automatic exposure, if the reticle illumination is unnecessarily bright, a photometric range index can not be aligned with the target so as to generate an error in the exposure time, thereby rendering an inappropriate exposure. Therefore, an operation for turning off the illumination of the reticle is required at the photographing time, which is troublesome. Moreover, in order to confirm the index provided on the reticle together with the sample image, it is required to illuminate the reticle with a color different from that of the light from the sample, and an operation for changing over illumination light sources for the light emission is also troublesome.